Uranium displays rare type of radioactivity.

The rarest of all observed radioactive event sinvolves the simultaneous decay of two neutrons within an unstable atomic nucleus to form two protons, accompanied by the emission to two beta particles (electrons) and two neutrinos. Now researchers have added the isotope uranium-238 to the handful of nuclei known to display "double-beta" decay.

Measurements of uranium-238's double-beta decay rate, however, indicate that this unusual process occurs about 100 times faster than predicted by calculations based on conventional theory. These results, if correct, suggest two possibilities: Either neutrinos, normally assumed to have no mass, actually have a certain mass, or theoretical calculations of the decay rates of heavy nuclei are somehow flawed.

"It's a very difficult calculation," acknowledges George A. Cowan of the Santa Fe (N.M.) Institute, a member of the team that made the uranium-238 measurements. The explanation of the results depends largely on "what error you should attribute to the theoretical calculations," he says. "If you take the error to be small, then the [experimental] results are surprising, and the theory permits you to account for a [faster rate] by giving the neutrino a mass."

Cowan and co-workers Anthony L. Turkevich and Thanasis Economou of the University of Chicago report their findings in the Dec. 2 PHYSICAL REVIEW LETTERS.

Like many unstable nuclei, uranium-238 normally decays by ejecting an alpha particle, thereby transforming itself into thorium-234, which has two fewer neutrons and two fewer protons. When double-beta decay occurs, urarium-238 turns into plutonium-238, leaving the total number of neutrons and protons in the nucleus unchanged.

To detect this extremely rare type of radiactivity, the researchers first had to find a uranium sample uncontaminated by human-made sources of plutonium-238, such as fallout from atmospheric nuclear explosions and residues from processing plutonium-238 for use in power generators on spacecraft.

"It's easy to find extraneous sources of plutonium-238," Cowan says. "You have only to go out and process a certain amount of soil in your backyard, and there's enough there to give you a signal."

Indeed, excessive contamination halted the original experiment to detect double-beta decay in uranium at the Los Alamos (N.M.) National Laboratory. The researchers then turned to a long-forgotten, pure supply of a uranium compound known as uranyl nitrate that had been stored undisturbed and protected from contamination for 33 years at the University of Chicago.

In 1956, "we had some money left over in one of our contracts, and we bought some uranyl nitrate, then forgot about it," Turkevich says. After abandoning the Los Alamos effort," rememebered this sample."

Working with 8.47 kilograms of the pure uranium salt, the researchers extracted and purified the plutonium present in the sample. By counting the number of alpha particles of a certain energy emitted by the purified plutonium, they could determined the number of plutonium-238 atoms present in the sample. Assuming that those nuclei represented the products of double-beta decay, the team could estimate the rate at which double-beta decay occurs in uranium.

Turkevich and his colleagues found a decay rate of about 0.1 count per day, which corresponds to a half-life for double-beta decay in uranium-238 of [10.sup.21] years. That rate is roughly 100 times greater than the best the theoretical estimate to date, Turkevich says.

"If this is true, it's a very interesting result," says Michael K. Moe of the University of California, Irvine, whose group in 1987 became the first to observe double-beta decay in the laboratory. Their experiments involved the isotope selenium-82.

However, Moe adds, theoretical calculation usually predict faster double-beta decay rates than those actually observed. Because the newly observed rate is considerably faster than the predicted rate, Moe suggests that Turkevich and his co-workers may have overlooked some background sources of plutonium-238 that would contribute to the observed decay rate.

"We took great pains," Cowan says. "We did the most careful background measurements we could. We tried to eliminate all extraneous sources of plutonium-238."

If both theory and experiment are correct, the observation of a faster decay rate implies that the neutrino has a mass, expressed in terms of energy, of about 14 electron-volts. This mass, however, is nearly 10 times larger than the value obtained from recent observations of double-beta decay in germanium-76.

Turkevich and his colleagues are now considering repeating their experiment. They recently learned of a supply of uranium left over from Germany's effort to develop nuclear weapons during World War II. Sealed up and stored for decades in Vienna, this uranium would likely be free of contamination from radioactive fallout.

"If we can find the resources to repeat the experiment, they've offered to let us have the uranium," Turkevich says. "We can't think of anything we've done wrong, but it would be good to try to repeat the experiment."